Mechanism and solvent dependence of the solvent-catalysed pseudo-intramolecular proton transfer of 7-hydroxyquinoline in the first electronically excited singlet state and in the ground state of its tautomer

Abstract

Picosecond time-resolved fluorescence of solutions of 7-hydroxyquinoline in neat alcohols has revealed two different excited-state tautomerization processes. There is a fast process which appears as the only one in the case of solutions in cyclohexane containing a small amount of alcohol. This fast process occurs within electronically excited 1:2 solute–alcohol complexes. The slower process is associated with tautomerization in excited solute–alcohol complexes which contain, at least initially, more than two solvent molecules. The size of the alcohol molecule determines the relative importance of the two processes in the phototautomerization.

The reverse double proton transfer process of the tautomer in its ground state has been observed by transient absorption spectroscopy. The groundstate tautomer decays monoexponentially with a rate constant depending both on the nature of the alcohol and on its concentration in the solution. In the limit of low alcohol concentrations, the rate constant correlates with the proton donor strength of the alcohol. The rate constant is temperature-dependent, and has a deuterium isotope effect which is temperature-independent. A previously suggested explanation in terms of a two-step process also applies here. The first step involves thermally activated solvent reorganization to achieve a proper conformation of a solute–alcohol complex. The second step is considered to be a thermally unassisted proton-tunnelling process in the complex with the proper conformation.